The disclosed embodiments relate to a method for producing a magnetic resonance radio frequency (RF) field in a magnetic resonance imaging system.
Magnetic resonance tomography scanners are imaging systems based on magnetic resonance measurement of nuclear spins. Magnetic resonance tomography scanners are well established and successful in versatile applications. In magnetic resonance image acquisition, a strong static main magnetic field B0 is used to initially align and homogenize magnetic dipoles to be examined. A de-phasing or relaxation time after a deflection of the magnetization from the initial alignment is established in order to determine material properties of an examination object to be imaged. Thus, different relaxation mechanisms or relaxation times, which are typical for the material, may be identified.
The deflection is achieved by a number of RF pulses, the so-called B1+ field, which are tuned to the Larmor frequency of the dipoles to be excited. For the purposes of differentiation, the magnetic field emitted by the body itself (after excitation by a B1+ field) is therefore referred to as B1− field. The Larmor frequency depends on the properties of the examined material and scales with the strength of the main magnetic field B0. Electromagnetic RF pulses having a Larmor frequency of 42.6 MHz and corresponding multiples thereof are often used for magnetic resonance measurements on biological examination objects. The value of 42.6 MHz corresponds with the Larmor frequency of proton spins in a main magnetic field B0 with a strength of 1 T. The multiplication factor corresponds with the strength of the main magnetic field B0 in T.
Ultra-high field magnetic resonance imaging scanners with a main magnetic field strength of 7 T have been developed. Such scanners provide an increase in the magnetic flux density, but with a significant increase in the power requirements of the transmission coils. In body-coil arrangements, the transmission power scales with the square of the strength of the main magnetic field. Therefore, if the strength of the main magnetic field is increased fourfold, the transmission power of the RF coil has to be increased by a factor of 16. On one hand, the increase leads to a significant increase in energy costs. On the other hand, the patient is exposed to a strong electromagnetic field, leading to a significant increase in a specific absorption rate (SAR) value. Moreover, the electromagnetic field produced by the body coil is not restricted to the examination region. Regions of the patient not to be examined are also exposed. Still further, the homogeneity of the RF field produced by a body coil in the examination object decreases with increasing magnetic field strength. The decrease may be traced back to dielectric effects within the examination object.